IMHO and believe me I'm no expert but with a 5.125 bore that is a lot of piston to be moving.
Here is some Hemi bore and stroke data
http://victorylibrary.com/mopar/rb-bs.htm
and some BBC data
http://billygrahamracing.com/BBC.asp
My biggest concern would be at that bore and being boosted is flame propagation going to be an issue?
Detonation will more than likely eat those custom pistons long before you can get the blow thru carb tuned.